1. Field of the Invention
The invention relates to a picture signal processor having a contrast reduction circuit to protect the processing circuits and the picture display tube from overload. The invention further relates to a picture display apparatus comprising the picture signal processor.
2. Description of the Related Art
Current television receivers comprise a variety of beam current limiters to protect the processing circuits and the picture display tube from overload. A long-term average beam current limiter protects against thermal overload so as to provide a longer picture tube life time. A short term average beam current limiter protects the line output transformer against saturation. The expression "long-term average beam current" relates to the maximum average beam current specified by the picture display tube manufacturer for pictures which are stationary for an indefinite period of time such as during the display of test pictures, computer images, teletext data or stationary television scenes lasting longer than 30 seconds. The expression "short-term average beam current" relates to the condition where the contents and intensity of the displayed image vary continuously such as during live television pictures. The circuits and the picture display tube are further protected from overload by a slow peak beam current limiter which protects against local dooming, i.e. heating of a part of the shadow mask tube due to stationary high intensity picture objects. Finally, a fast peak beam current limiter protects against spot blooming and clipping of the video output amplifiers. All these beam current limiters operate through the existing contrast controller, and if this is not sufficient, through the brightness controller.
The integrated circuit TDA 4680, described in the Philips Components pamphlet "Die Video-Prozessor-Schaltung TDA 4680, Technische Informationen 900503", comprises a peak and average beam current limiter, see section 2.5 of the pamphlet. The peak beam current limiter operates as follows. The voltage across a capacitor forms the setting voltage for the peak beam current limiting. This setting voltage is multiplied by a value which represents the contrast desired by the user. When the beam current is within its admissible peak value range, the capacitor is charged up to a charge voltage of about 4 V. When the peak beam current becomes too large, a current source is activated to rapidly discharge the capacitor with a discharge current exceeding 4 mA. A corresponding contrast reduction and, when necessary, brightness reduction is achieved until the peak beam current has returned to within its admissible range. When the signal peak amplitude is reduced, the capacitor is recharged with a charge current of about 1 .mu.A which is substantially smaller than the above-mentioned discharge current. Consequently, it lasts for several picture periods until the contrast and brightness settings resume the values desired by the user after disappearance of a large peak value in the video input signal. This is deemed to be necessary, because large peak values commonly appear punctually in the displayed picture, so that the obtained contrast and brightness reduction may not have changed substantially when the large peak values reappear in a subsequent field period. The contrast and brightness settings are thus influenced by three entities, viz. the setting asked for by the user, the setting determined by the average beam current limiter, and the setting determined by the peak beam current limiter. The smaller of the settings determined by the average and peak beam current limiters is multiplied by the setting asked for by the user to obtain the final contrast setting.
The following problems are observed in such prior art television receivers. 1) Since the overall picture contrast is reduced, the whole picture becomes soft, possibly due to only a single peak excursion in the entire picture. 2) Due to changing signal contents, the contrast will exhibit an annoying "pumping", i.e. it is constantly going up and down. Since the contrast regulating range is very large, this is clearly visible. The latter behavior is exaggerated by the habit of factory preprogramming the contrast to a maximum: during prolonged large scenes, the contrast will always move up to its maximum. 3) The contrast control circuitry is responsive to many other signals than the user contrast control signal, and in consequence as long as any limiting operation occurs, the upper range of the contrast control seems to be "dead" in a manner not understood by the user.
U.S. Pat. No. 4,712,132 describes another device for reducing the amplitude range, i.e. the contrast of signals representing an image. The device comprises a circuit for determining at each instant a correction signal whose instantaneous value is a predetermined non-linear function of the present input color signal having the highest value at that instant. For each of the three color signals, the device further comprises a separate multiplier which multiplies the corresponding color signal by the correction signal. The device has the drawback that the non-linear function does not sufficiently ensure that a given maximum admissible amplitude of the color signal is not exceeded.